Corrosion Inhibiting Coating Based on Cerium Oxide and a Catecholic Polymer

ABSTRACT

A method for prevention of corrosion of a metal object comprises applying a coating to the metal object. Thee coating comprises at least one cerium oxide and at least one polymer. The at least one polymer comprises at least one catecholic component covalently bound thereto and displays a net positive charge at a pH of 7.

TECHNICAL FIELD

The present invention relates generally to an environmentally friendlycoating for the prevention of corrosion of metals as well as a methodfor applying the coating.

BACKGROUND

It is known that cerium oxide treatment of a metal improves thecorrosion resistance due to the formation of a protective oxide filmwhich acts as an active protective layer on the metal surface.

Adhikari et al in Electrochimica Acta, Vol 53, issue 12, pp 4239-4247studies anticorrosion properties of a coating comprising modifiedpolyaniline dispersed in polyvinylacetate on carbon steel.

Zhitomirsky in Surface Engineering, Vol 20, issue 1, pp. 43-47 discloseselectrodeposition of films comprising ceria and the cationic polymerpolyethylenimine.

Corrosion inhibiting coatings according to the state of the art oftenuse compounds which are known to cause environmental problems and/orhealth problems for users. Examples include chromium compounds.

Y. Gao et al in Transactions of the Institute of Metal Finishing vol 84,no 3, 2006, pp 141-148 discloses corrosion protection of zincelectroplated steel. The corrosion inhibiting coating is a coatingcomprising either gelatine or albumin as well as dichromate. Also analternative coating comprising gelatin and cerium trichloride isdisclosed. It is concluded that the ability of cerium trichloride tostabilize protein formulations against putrefaction is questionable andthat its adoption would require an associated stabilizer.

US 2004/0028820 discloses coating of aluminum using cerium ions in thepresence of an oxidizing agent. The preferred cerium-based coatingscomprise cerium oxide, hydrated cerium oxide, or forms of ceriumhydroxide after coating. The coating bath optionally contains animalgelatin, glycerol, or other organic additive to improve coatinguniformity and corrosion resistance. It is speculated that the gelatinfunctions to modify the nucleation and growth sites.

Mussel adhesive protein (MAP) is formed in a gland in the foot of byssusforming mussels, such as the common blue mussel (Mytilus edulis). U.S.Pat. No. 5,015,677 as well as U.S. Pat. No. 4,585,585 disclose that MAPhas very strong adhesive properties after oxidation and polymerization,e.g. by the activity of the enzyme tyrosinase, or after treatment withbifunctional reagents.

J. H. Waite et al in The Journal of Adhesion, vol. 81, 2005, pp 297-317reviews adhesive proteins from mussels.

Lee et al in Science, vol 318, 2007, pp 426-430 discloses dopamineself-polymerization to form thin, surface-adherent polydopamine filmsonto a wide range of inorganic and organic materials, including noblemetals, oxides, polymers, semiconductors, and ceramics.

WO 03/008376 discloses conjugation of DOPA moieties to various polymericsystems.

A. Statz et al in Biofouling, vol 22, no 6, 2006, pp 391-399 concernsmarine antifouling and fouling-release performance of titanium surfacescoated with a polymer consisted of methoxy-terminated poly(ethyleneglycol) conjugated to the adhesive amino acid DOPA and was chosen basedon its successful resistance to protein and mammalian cell fouling. Itis concluded that this polymer may be effective in marine antifoulingand fouling-release applications.

CN 101658837 discloses preparation of an anticorrosive film for metalsurfaces. The film comprises dopamine.

WO 03/080137 discloses a method for attaching two surfaces using aprotein and periodate ions.

In the prior art there is still a need for an improved corrosionprotection.

SUMMARY

It is an object of the present invention to alleviate at least some ofthe disadvantages of the prior art and to provide an improved coatingfor at least partially preventing corrosion of metals.

In a first aspect there is provided a coating for metal objects, saidcoating comprising at least one cerium oxide and at least one polymer,wherein the at least one polymer comprises at least one catecholiccomponent covalently bound thereto, and wherein the at least one polymerdisplays a net positive charge at a pH of 7.

In a second aspect there is provided a method for coating a metalobject, said method comprising the step of applying at least one ceriumoxide and at least one polymer, wherein the at least one polymercomprises at least one catecholic component covalently bound thereto,and wherein the at least one polymer displays a net positive charge at apH of 7.

Further aspects and embodiments are described in the appended claims.

Advantages of the invention include that the material is environmentalfriendly and does not display the serious health risks as the compoundsaccording to the state of the art. Further an excellent corrosioninhibition is obtained. Moreover only small amounts of coating materialis required.

The combination between the polymer and small particles comprisingcerium oxide gives the excellent corrosion protection.

The polymer displays a strong binding to the surface. The combination ofmaterials, i.e. the polymer and small particles of cerium oxide isfavorable since the small particles and the MAP protein form a compactcomposite film. Thus there is a synergistic effect of cerium oxide andthe polymer.

A further advantage is that the composite film grows together with thecorrosion product, as evidenced by the increase in the protectionefficiency with time.

The corrosion inhibiting properties are excellent for many metals andeven for carbon steel.

There is further the advantage that it is possible to build up thickerfilms for instance by several depositions of the polymer and the smallparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described with reference to the following drawings inwhich:

FIG. 1 shows the sensed mass as a function of deposition number duringalternative adsorption of MAP (filled squares) and ceria nanoparticles(filled circles) and after rinsing with water after the MAP (emptysquares) and ceria nanoparticle (empty circles) deposition steps.

FIG. 2 shows the change in energy dissipation as a function ofdeposition number during alternative adsorption of Mefp-1 (filledsquares) and ceria nanoparticles (empty circles).

FIGS. 3a and 3b show optical microscope images of 500 timesmagnification for the MAP and ceria nanoparticle composite films (4adsorption steps for MAP and 4 for ceria nanoparticles), on carbonsteel, and on silica, respectively.

FIG. 4a shows a tapping mode AFM topography image of a compact part ofthe composite film on silica, FIG. 4b shows a corresponding phase image,showing a densely packed uniform nanostructure of the film, and FIG. 4cshows a single line height profile showing nano-sized particles.

FIG. 5a shows a tapping mode AFM topography image of the compact part ofthe composite film on carbon steel surface, and FIG. 5b shows acorresponding phase image, showing two distinct phases in thecomposition.

FIGS. 6a-6d show Bode plots for a sample with a composite filmconsisting of 4 alternating MAP/ceria layers, as compared to carbonsteel control sample and the sample with MAP in the solution afterdifferent periods of exposure time, 1 hour (FIG. 6a ), 1 day (FIG. 6b ),3 days (FIGS. 6c ), and 7 days (FIG. 6d ).

FIG. 7 shows polarization curves obtained after 7 days of exposure in0.1 M NaCl solution with 0.2 M H₃PO₄ at pH 4.6, for carbon steel withoutprotection (control), with 100 ppm MAP added as inhibitor (MAP), withthe MAP and ceria composite film (MAP+ceria). The curve for stainlesssteel 316L obtained immediately after immersion is included forcomparison.

FIG. 8 shows OCP vs. time for Zn sample with the MAP and ceria compositefilm in 0.1 M NaCl solution with 0.2 M H₃PO₄ at pH 4.6, in thebeginning, after 1 hour, 1, 3 and 7 days of exposure.

FIG. 9 shows Bode plots of Zn sample with the MAP and ceria compositefilm in 0.1 M NaCl solution with 0.2 M H₃PO₄ at pH 4.6, after 1 hour, 1,3 and 7 days of exposure.

FIGS. 10a-10d show Bode plots of Zn sample with the MAP and ceriacomposite film and the control sample (no film) in 0.1 M NaCl solutionwith 0.2 M H₃PO₄ at pH 4.6, after 1 (FIG. 10a ) hour, 1 day (FIG. 10b ),3 days (FIGS. 10c ), and 7 (FIG. 10d ) days of exposure.

DETAILED DESCRIPTION

Before the invention is disclosed and described in detail, it is to beunderstood that this invention is not limited to particular compounds,configurations, method steps, substrates, and materials disclosed hereinas such compounds, configurations, method steps, substrates, andmaterials may vary somewhat. It is also to be understood that theterminology employed herein is used for the purpose of describingparticular embodiments only and is not intended to be limiting since thescope of the present invention is limited only by the appended claimsand equivalents thereof.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a”, “an” and “the” include plural referentsunless the context clearly dictates otherwise.

If nothing else is defined, any terms and scientific terminology usedherein are intended to have the meanings commonly understood by those ofskill in the art to which this invention pertains.

The term “about” as used in connection with a numerical value throughoutthe description and the claims denotes an interval of accuracy, familiarand acceptable to a person skilled in the art. Said interval is ±20%.

As used throughout the claims and the description, the term “metalobject” denotes an object comprising at least partially a metal surface.An object made of a metal and a non-metal where a part of the surface isa metal surface is thus encompassed within the term metal object.Further objects at least partially made of different metals as well asmetal alloys are encompassed within the term.

As used throughout the claims and the description, the term “coating”denotes a covering that is applied at least partially to the surface ofan object.

As used throughout the claims and the description, the term“polypeptide” denotes polymers formed of amino acid residues. Proteinsare encompassed within the term polypeptide. Polypeptides comprising 50or more amino acid residues are also denoted proteins.

As used throughout the claims and the description, the term “ceriumoxide” denotes a chemical compound or complex comprising the chemicalelement cerium (Ce) and the chemical element oxygen (O). The term“cerium oxide” denotes oxides of cerium including Ce₂O₃ and CeO₂. Theterms ceric oxide, ceria, cerium(III) oxide, cerium(IV) oxide and ceriumdioxide are also encompassed by the term cerium oxide.

As used throughout the claims and the description, the term “marineorganism” denotes water living organisms.

As used throughout the claims and the description, the term “mollusc”denotes the phylum mollusca of invertebrate marine animals.

As used throughout the claims and the description, the term “mussel”denotes several families of the bivalvia mulluscs including the familymytilidae.

As used throughout the claims and the description, the term “byssusforming mussels” denotes bivalvia molluscs forming byssus.

As used throughout the claims and the description, the term “carbonsteel” denotes alloys comprising more than 50 wt % iron and with acarbon content of less than 2 wt %. Steel is considered to be carbonsteel when no minimum content is specified or required for chromium,cobalt, molybdenum, nickel, titanium, tungsten, vanadium and zirconiumor any other element to be added to obtain a desired alloying effect.

In a first aspect there is provided a coating for metal objects, saidcoating comprising at least one cerium oxide and at least one polymer,wherein the at least one polymer comprises at least one catecholiccomponent covalently bound thereto, and wherein the at least one polymerdisplays a net positive charge at a pH of 7.

The net charge of the polymer often varies with the pH depending on thenature of the polymer. For groups of the polymer may have a charge whichvaries with the pH. The pH of the polymer is positive at the applicationof the polymer. At pH 7 there may be both positive and negative chargeson the polymer, but the net charge of a polymer is positive.

In one embodiment the at least one cerium oxide is CeO₂ (ceria). In oneembodiment the at least one cerium oxide is in the form of particles. Inone embodiment the at least one cerium oxide is in the form of particleswith a diameter of 1-1000 nm. It is an advantage to use particles withrelatively small diameter. Examples of further size intervals for theparticles include but are not limited to 4-80 nm, 4-40 nm, 5-50 nm, and5-100 nm. Without wishing to be bound by any particular scientifictheory the inventors believe that the particles of cerium oxide and thepolymer form a composite layer and a dense coating with suitableproperties.

In one embodiment the at least one cathecholic component is at least oneselected from DOPA (L-3,4-dihydroxyphenylalanine), and aDOPA-derivative.

In one embodiment the at least one polymer comprises at least 2 wt %based on the molecular weight Mw of at least one moiety selected fromDOPA (L-3,4-dihydroxyphenylalanine), and a DOPA-derivative. In oneembodiment the at least one polymer comprises at least 5 wt % based onthe molecular weight Mw of at least one moiety selected from DOPA(L-3,4-dihydroxyphenylalanine), and a DOPA-derivative. In anotherembodiment 6-30 wt % of the polymer based on the molecular weight Mw areat least one moiety selected from DOPA (L-3,4-dihydroxyphenylalanine),and a DOPA-derivative.

In one embodiment the at least one polymer is at least one polypeptideextracted from a byssus-forming mussel. In one embodiment the at leastone polypeptide comprises 30-3000 amino acid residues and tandemlylinked peptide repeats comprising 3-15 amino acid residues each. In oneembodiment 6-30 wt % of the number of amino acid residues in apolypeptide are L-3,4-dihydroxyphenylalanine (DOPA). In one embodiment2-4 wt % of the number of amino acid residues in a polypeptide areL-3,4-dihydroxyphenylalanine (DOPA). In one embodiment at least 3 wt %of the number of amino acid residues in a polypeptide areL-3,4-dihydroxyphenylalanine (DOPA). In one embodiment 10-15 wt % of thenumber of amino acid residues in a polypeptide areL-3,4-dihydroxyphenylalanine (DOPA). In one embodiment 20-30 wt % of thenumber of amino acid residues in a polypeptide areL-3,4-dihydroxyphenylalanine (DOPA). In one embodiment the polypeptideis a protein extracted from a byssus-forming mussel, such protein iscalled MAP (mussel adhesive protein). In one embodiment the polymer isat least one protein selected from the group consisting of MEFP-1,MEFP-2, MEFP-3, MEFP-4, and MEFP-5. The abbreviations stand for MytilusEdulis, foot protein 1, 2, 3, 4, and 5 respectively. In one embodimentthe polypeptide is MEFP-1.

In one embodiment the polymer a poly(alkyleneoxide) co-polymer. In oneembodiment the polymer is a co-polymer of ethylene oxide and ahydrophobic co-monomer. In one embodiment said hydrophobic co-monomer isselected from the group consisting of propylene oxide, lactic acid,glycolic acid and caprolactone. In one embodiment said co-monomercomprises a hydrophobic block, and said polymeric component is a blockco-polymer.

In one embodiment at least two catecholic components are conjugated tothe polymer.

In one embodiment the coating comprises at least one layer comprisingthe at least one polymer, and wherein the coating further comprises atleast one layer comprising the at least one cerium oxide. In anotherembodiment the coating comprises two or more layers comprising the atleast one polymer, and wherein the coating further comprises at leasttwo or more layers comprising the at least one cerium oxide.

In one embodiment the coating is at least partially applied to a metalobject. In one embodiment the metal is at least one metal selected fromthe group consisting of iron, zinc, aluminum, and copper. In anotherembodiment the metal is steel. In yet another embodiment the metal iscarbon steel.

Without wishing to be bound by any particular theory the inventorsbelieve that the oxidizing ability of the MAP and ceria, can form aprotective oxide (e.g., Fe₂O₃) on carbon steel surface beneath (orincorporated into) the composite film.

In one embodiment the coating is at least partially applied to an objectcomprising carbon steel.

In a second aspect there is provided a method for coating a metalobject, said method comprising the step of applying at least one ceriumoxide and at least one polymer, wherein the at least one polymercomprises at least one catecholic component covalently bound thereto,and wherein the at least one polymer displays a net positive charge at apH of 7.

In one embodiment the method comprises the steps of: a) applying atleast one layer comprising the at least one cerium oxide, and b)applying at least one layer comprising the at least one polymer. In oneembodiment steps a) and b) are performed in one step so that the atleast one cerium oxide and the at least one polymer are applied in onestep. In one embodiment a layer comprising the at least one ceriumoxide, and a layer comprising the at least one polymer are appliedsequentially several times. It is an advantage of the invention thatseveral layers can be made. In this way it is possible to control thelayer thickness. A thicker coating comprising several layers offers amore resistant coating.

In one embodiment the application is performed using at least one methodselected from the group consisting of spraying and dipping.

In one embodiment the polypeptide is oxidized during the procedure. Inone embodiment the polypeptide is oxidized by addition of an oxidant. Inone embodiment the polypeptide is oxidized using periodate ions. In oneembodiment the polypeptide is oxidized by increasing the pH to 8 orabove.

In one embodiment the polymer is cross-linked. The oxidation andcross-linking creates excellent adhesion and covalent bonds between thepolymer chains and to oxides on the surface as well as to the particlescomprising cerium oxide.

In a third aspect there is provided a liquid coating composition formetal objects comprising at least one cerium oxide, and at least onepolymer, wherein the at least one polymer comprises at least onecatecholic component covalently bound thereto, and wherein the at leastone polymer displays a net positive charge at a pH of 7.

In one embodiment the at least one cathecholic component is at least oneselected from DOPA (L-3,4-dihydroxyphenylalanine), and aDOPA-derivative.

The liquid coating composition is intended for coating a metal object asdescribed above.

In a fourth aspect there is provided a kit comprising at least onecerium oxide, an instruction to coat a metal, and at least one polymer,wherein the at least one polymer comprises at least one catecholiccomponent covalently bound thereto, and wherein the at least one polymerdisplays a net positive charge at a pH of 7.

In one embodiment the kit comprises a first liquid coating compositionand a second liquid coating composition, wherein said first liquidcoating composition comprises said at least one polymer, and whereinsaid second liquid coating composition comprises said at least onecerium oxide. In such an embodiment it is intended that the two coatingcompositions are applied sequentially in any order. The two coatingcompositions are in one embodiment applied sequentially several times.

In an alternative embodiment the kit comprises a liquid coatingcomposition, wherein said liquid coating composition comprises said atleast one polymer and said at least one cerium oxide. In such anembodiment it is intended that the coating composition is applied. Inone embodiment the coating composition is applied several times.

In one embodiment in the above kit the at least one cathecholiccomponent is at least one selected from DOPA(L-3,4-dihydroxyphenylalanine), and a DOPA-derivative.

In a fifth aspect there is provided a metal object coated with thecoating described above.

In a sixth aspect there is provided use of at least one cerium oxide andat least one polymer, wherein the at least one polymer comprises atleast one catecholic component covalently bound thereto, and wherein theat least one polymer displays a net positive charge at a pH of 7, forthe prevention of corrosion of metals.

In an seventh aspect there is provided use of at least one cerium oxideand at least one polymer, wherein the at least one polymer comprises atleast one catecholic component covalently bound thereto, and wherein theat least one polymer displays a net positive charge at a pH of 7, forthe coating of metals.

As evidenced from the examples also the MAP protein itself provides somecorrosion inhibition.

Without wishing to be bound by any scientific theories the inventorsbelieve that the presence of L-3,4-dihydroxyphenylalanine (DOPA), isresponsible for both adhesive and crosslinking characteristics as wellas the hardening properties of the polymer.

EXAMPLES Example 1

MAP (more specifically Mefp-1) used for the experiments was supplied byBiopolymer Products AB (Gothenburg, Sweden). The MAP was delivered in0.2 M H3PO4 solution at a concentration of 18.7 mg/ml. The solution wasstored at 4° C.

The ceria (cerium oxide) nanoparticles (NANOBYK-3810) were supplied byBYK Company, Germany. The diameter of the ceria particles is 10nanometers with a narrow size distribution according to the supplier.The particles are dispersed in water and stored at room temperaturebefore use. All chemicals used for preparing solutions were ofanalytical grade and the water was of high purity. The solutions weresonicated for 10 minutes before the experiment to ensure good dispersionof nanoparticles.

The carbon steel used as substrate was cold rolled low carbon steel (DC01, 1.0330), supplied by IVF, Sweden. The steel sheet samples were wetground with SiC grinding paper successively to 1200 grids, and thencleaned ultrasonically with ethanol. For the AFM measurements, thesample surface was first ground using SiC grinding paper in severalsteps down to 2400 grits, and then a final polishing procedure wasperformed by using a suspension of 0.02 ?m alumina particles. Afterwardthe sample was cleaned ultrasonically with ethanol and dried with agentle stream of nitrogen gas.

For the study of morphology of the MAP and ceria composite film, silicawas also used as an inert model substrate. Thermally oxidized siliconwafers were purchased from Wafer Net, Germany. The wafers were cut torequired size (1 cm×1 cm). Prior to the experiment, the silica surfacewas ultrasonically cleaned with ethanol and dried with a stream ofnitrogen gas.

The QCM instrument used was a q-sense E4 microbalance (q-sense,Gothenburg). It was employed to follow the film formation process onquartz crystal coated with a thin stainless steel-like layer (q-sense,Gothenburg). The composition analysis of the coated surface layerreveals a high content of Cr and oxygen, indicating an oxidized Crsurface, which was sufficiently inert to provide a stable baselineduring the measurement.

MAP and ceria solutions were prepared for the deposition of thecomposite film: 100 ppm MAP in 1% citric acid with 50 mM NaCl at pH 6,and 500 ppm ceria nanoparticles dispersed in water with 50 mM NaCl. Thesubstrate sample was firstly immersed for 1 hour in the MAP solution andthen 40 minutes in the nano-ceria solution, respectively, and thisprocedure was repeated 4 times to deposit the composite film of thesample surface. The immersion procedures were carried out at roomtemperature and the solutions were renewed between each step. After allthe deposition procedures, the sample was gently rinsed in pure waterand dried. The sample was kept in air at room temperature overnightbefore the corrosion test.

A QCM-D instrument, which is a highly sensitive balance based on themeasurement of changes in the resonance frequency of a quartz crystaloscillator, was used to study the film formation process. Adsorption (ordesorption) of the material to the crystal surface will give rise to afrequency change, which is measured and used to calculate the adsorbedamount according to the linear relationship described by the Sauerbreyequation (assuming a rigid adsorbed layer).

${\Delta \; m} = \frac{\Delta \; f \times C}{n}$

where Δf is the measured frequency change due to adsorption, C is themass sensitivity constant of the quartz crystal, 17.7 mg*m⁻²*Hz⁻¹ forthe 5 MHz resonance frequency, and n is the overtone number (n=1, 3, 5 .. . ). This formula was used for evaluating the experimental data.

The QCM-D also gives information about shear viscoelastic properties ofthe film by measuring the energy dissipation (D). This parameter isobtained from the rate of decay of the crystal oscillation when thevoltage is switched off. For a soft film the decay time is small, andthe dissipation value is high, whereas for a rigid film the dissipationvalue is smaller.

An optical microscope was used to inspect a large surface area.Moreover, a Nanoscope Multimode AFM was used to image the detailedmorphology of the composite films deposited on the carbon steel surfaceand the silica surface. The probe was a phosphorus doped n-type silicontip with a spring constant of 5.7 N/m and resonant frequency of 160 kHz.All images were taken in tapping mode with a scan rate of 1 Hz, and wereflattened to remove the slope due to sample tilting. The optical and AFMimaging were done in air, ex-situ, on dried samples.

AFM imaging by tapping mode yields both a topography image (height) anda phase image. The phase image is influenced by variations in surfacecomposition, adhesion, friction, viscoelasticity, etc. Phase imagesobtained simultaneously with height images give additional informationof the microstructure.

The electrochemical measurements were performed for samples exposed to0.1 M NaCl solution with 0.2 M H3PO4, and the pH was adjusted to 4.6using a NaOH solution. EIS measurements were performed to determine thepolarization resistance, a measure of corrosion resistance, for samplescoated with the composite film, and for the control sample without anyfilm. For comparison, the carbon steel sample exposed to the samesolution with 100 ppm MAP as corrosion inhibitor was also included inthis study. The EIS measurements were carried out at the open-circuitpotential after 1 hour, 1, 3 and 7 days of exposure with perturbationamplitude of 10 mV and over the frequency range from 104 Hz to 10−2 Hz.

Upon termination of the exposure, potentiodynamic polarization wasperformed to further evaluate the corrosion protection properties of thecomposite film. The potential sweep was started at −0.2 V vs.open-circuit potential, and terminated at an anodic potential at whichthe current density reached 1 mA/cm². The sweep rate was 10 mV/min.

For monitoring the film formation process, the “stainless steel” coveredQCM crystals were alternately immersed in 100 ppm MAP (positivelycharged) and 500 ppm ceria nanoparticle (negatively charged) solutionscontaining 1 wt % citric acid at pH 6 and 50 mM NaCl. After eachdeposition immersion the excess of material as well as the NaCl wereremoved by rinsing with water. The adsorption experiments were doneusing a continuous flow with a flow rate 100 microL/min and at atemperature of 23° C. Before the experiment the base line in the buffersolution was established.

The growth of the composite film was followed by the mass change as afunction of deposition number as shown in FIG. 1. Filled symbolscorrespond to the sensed mass (the mass sensed by the QCM is due to boththe deposited material and that of hydrodynamically coupled water)during the adsorption step, and open symbols to that obtained afterrinsing with water. The data demonstrate a linear increase of the masswith the layer number. The results suggest a continuous build-up of thecomposite film by increasing number of immersion steps, and the ceriananoparticles are irreversibly (with respect to dilution) incorporatedinto the composite film.

The change in energy dissipation as a function of deposition number forMAP and ceria nanoparticle deposition is shown in FIG. 2. The highdissipation values obtained after MAP adsorption (odd layer numbers)demonstrate that MAP is adsorbed in extended conformations that allowsignificant hydrodynamic coupling to the solvent. In contrast, the lowvalues obtained after ceria nanoparticle adsorption (even layer numbers)demonstrate formation of a more rigid layer.

FIG. 3 shows examples of optical images at 500 times magnification ofthe MAP and ceria nanoparticle composite films deposited on the carbonsteel and on silica, respectively. It can be seen that the morphology ofthe MAP and ceria nanoparticle composite films are similar on bothsurfaces. The films are not uniform, and there are micro-domainsextending from and randomly distributed in the compact and smoothsurface layer. The compact and smooth parts of the film consist ofnanostructures as revealed by AFM (below). It appears that the MAP andceria that are present in the composite film do not form separatelayers, but rather MAP binds the ceria nanoparticles and together form afractal-like structure consisting of extending domains and compactdomains.

An example of topography and phase images, obtained by tapping mode, ofthe compact part of the composite film on a silica surface, as well as asingle line height profile are shown in FIG. 4. The topography image andthe line height profile clearly show nano-sized particles. Although manyparticles appear to have a size of about 20-40 nm, the smallestparticles have a spherical shape and a size of ca. 10 nm, which is thesize of the ceria nanoparticles used. The larger particles could beaggregates of the ceria nanoparticles glued together by MAP. Thecorresponding phase image indicates a densely packed uniformnanostructure of the film.

The MAP and ceria nanoparticle composite film in this study issignificantly more compact than the adsorbed MAP film formed in anotherstudy from a solution with 1 mg/ml (10 times higher than used in thiswork) at pH 4.6 on silica. Without wishing to be bound by any particularscientific theory the inventors speculate that highly charged cations anin particular ceria (Ce³⁺) may promote the adsorption of MAP.

FIG. 5 shows an example of topography and phase images, obtained bytapping mode, of the compact part of the composite film on carbon steelsurface. The detailed nanostructure of the composite film on carbonsteel is different from that on silica (FIG. 4). The carbon steelsurface was fully covered by aggregates with a size about 100 nm, whichis significantly larger than the ones formed on the silica surface.Moreover, as revealed in the phase image, each large aggregate consistsof two different phases, indicating different properties of thecomponents of the aggregates. It can be expected that the hard ceriananoparticles and soft MAP components provide the contrast in the phaseimage. Although it is not possible to ascertain the harder or softercomponents by the phase image, the inventors speculate that the lighterphase may be ceria nanoparticles as judged by their small size of about10 nm, the darker phase is probably associated with MAP or MAP-metalcomplexes. A densely packed MAP and ceria nanoparticle composite filmfully covering the surface should give a high corrosion protection forcarbon steel, which indeed is verified in the electrochemicalmeasurements (next section).

Without wishing to be bound by any particular scientific theory theinventores speculate that, based on the AFM observation, it may besuggested that, on the carbon steel surface where Fe ions are released,complexation of MAP and metal ions takes place and this results information of large aggregates consisting of ceria nanoparticles andMAP-metal complexes.

Typical EIS spectra in Bode form obtained after 1 hour, 1, 3 and 7 daysof exposure to 0.1 M NaCl solution with 0.2 M H₃PO₄ at pH 4.6 aredisplayed in FIG. 6. The results obtained for carbon steel with the MAPand ceria nanoparticle composite film are compared to those for barecarbon steel (control), and carbon steel with 100 ppm MAP added into thesolution. The impedance modulus at the low frequency limit gives anindication of the level of the polarization resistance. A higherpolarization resistance implies a higher corrosion resistance.

The results show clearly that the MAP and ceria composite film leads toa significantly increased corrosion resistance, already during theinitial period of exposure (1 hour). This protection effect is greatlyenhanced after 1 day's exposure, it continues to increase after 3 day'sexposure, and approaches a high level after 1 week's exposure. Incontrast, MAP added to the solution provides pronounced inhibitioneffect only after 1 week's exposure. The corrosion resistance of thecomposite film is more than one order of magnitude higher than thatgiven by the MAP inhibitor alone, clearly displaying the synergisticeffect.

The polarization curves obtained after termination of the 1-weekexposure are displayed in FIG. 7. The polarization curve of a stainlesssteel (316L) is also included in the figure for comparison. Analysis ofthe polarization curve for the composite film gives a corrosion currentdensity of the order of μA/cm², and the curve exhibits a small potentialrange of passivity. The results demonstrate that the MAP and ceriananoparticle composite film can provide an excellent corrosionprotection to carbon steel, which is almost comparable with that ofstainless steel.

By curve fitting of the small potential range around the corrosionpotential (activation control) of the polarization curves using theCorrView software, the corrosion current was obtained for these samplesand the data are shown in Table 1. It can be seen that the corrosioncurrent decreased ca. 7 times by the MAP inhibitor and ca. 70 times bythe MAP and ceria nanoparticle composite film, further illustrating thesynergistic effect. The current density of the composite film is around1 ?A/cm2, which in practice is often regarded as the level of passivityof alloys like stainless steels.

TABLE 1 Corrosion potential and current density obtained from thepolarization curves. Material E_(corr) (mV vs. Ag/AgCl) I_(corr)(μA/cm⁻²) 316L −290 0.24 MAP + Ceria −600 ± 30 0.86 ± 0.40 MAP −654 ± 2 7.90 ± 1.00 Control −647 ± 18 58.91 ± 6.84 

Example 2

Pure Zn was used as the substrate metal. The same pure MAP and ceriananoparticles were used as those used in example 1.

Zn sample surfaces were wet ground successively with sandpaper of 500,800, 1200 grits, after cleaning, the samples were left overnight in aclosed container. The deposition of the composite film was carried outon the next day.

Fresh MAP solution was prepared 2 min before the immersion. For the filmdeposition, the MAP solution contains 0.1 mg/mL MAP, 1% citric acid and50 mM NaCl, and the pH was 6. The ceria solution contains 500 ppm ceriananoparticles dispersed in water, and 50 mM NaCl. The film depositionprocedure was the same as usual: 1 hour immersion in the MAP solutionand 40 min in the ceria solution, without rinsing with water in between.The film deposition was performed by alternating immersion in the MAPsolution and ceria solution for 4 times.

It was noted that, at the beginning of second immersion in the MAPsolution, some reaction product was observed as a grayish layer floatingat the surface of the MAP solution. The floating product started toappear on the MAP solution after the first deposition of ceria, and itwas observed at all further steps for film deposition in the MAPsolution.

The Zn samples with the deposited MAP and ceria nanoparticle compositefilm were exposed to 0.1 M NaCl solution with 0.2 M H₃PO₄ at pH 4.6, theopen circuit potential (OCP) was recorded continuously for 15 minutes,and then EIS was performed after, 1 hour, 1, 3 and 7 days of exposure,as for the carbon steel samples (example 1).

The results from 3 parallel measurements show a good reproducibility, soonly the results from one set of samples are presented in this report.

FIG. 8 shows the OCP vs. time for the Zn sample with the MAP and ceriacomposite film in the beginning, after 1 hour, 1, 3 and 7 days ofexposure. In the beginning, the OCP of the sample was at ca. −1.1 V vs.Ag/AgCl, which is similar to Zn without any surface film. This indicatesthat the MAP and ceria composite film is permeable to the electrolyte.The OCP slightly increased after 1 day, indicating some change hasoccurred in the surface film. It follows that the OCP increasedsignificantly with time, reaching ca. −0.7 V after 3 days and −0.6 Vafter 7 days. It is clear that, during the exposure, some interactionstake place in the composite films and/or between the film and Zncorrosion products, which lead to a pronounced ennoblement of the Znsurface. Consequently, this resulted in an enhanced corrosion protectionof Zn in the solution, as confirmed by the EIS measurements.

FIG. 9 shows typical Bode plots of the EIS spectra obtained for the Znsample with the MAP and ceria composite film after 1 hour, 1, 3 and 7days of exposure. As can be seen from the EIS spectra, with a prolongedexposure, the sample increasingly exhibits capacitive behavior, and theimpedance at low frequency end (a measure of corrosion resistance)increased about two orders of magnitudes. The results suggest a greatenhancement in the corrosion resistance of the sample, which impliesthat the surface layer (composite film and corrosion products) becomesmore protective with the exposure.

It should be mentioned that, Zn is an active metal, usually in acidicsolutions or NaCl solutions it will corrode fast because no stablecorrosion products will form on the surface, and the OCP remains at thelow level due to the dominating electrochemical corrosion reaction ofZn. Based on the results above, it can be concluded that the MAP andceria composite film has a protection mechanism for zinc substrate inthe solution.

The figures (FIG. 10a-10d ) show comparison between the EIS results fromthe Zn sample with the MAP and ceria composite film and those from thecontrol sample without any film. Apparently the corrosion resistance ofthe control sample also increased with exposure in this solution. Theinventors speculate that this could be a result of interaction betweenZn and phosphoric acid present in the solution, similar to a phosphatetreatment.

What is claimed is:
 1. A method for prevention of corrosion of a metalobject, comprising applying a coating to the metal object, the coatingcomprising at least one cerium oxide and at least one polymer, whereinthe at least one polymer comprises at least one catecholic componentcovalently bound thereto, and wherein the at least one polymer displaysa net positive charge at a pH of
 7. 2. The method according to claim 1,wherein the at least one cerium oxide is CeO₂.
 3. The method accordingto claim 1, wherein the at least one cerium oxide is in the form ofparticles with a diameter of 1-1000 nm.
 4. The method according to claim1, wherein the at least one cathecholic component is at least oneselected from DOPA (L-3,4-dihydroxyphenylalanine), and a DOPAderivative.
 5. The method according to claim 1, wherein the at least onepolymer comprises at least 2 wt % based on the molecular weight Mw of atleast one moiety selected from DOPA (L-3,4-dihydroxyphenylalanine), anda DOPA-derivative.
 6. The method according to claim 1, wherein 6-30 wt %of the polymer based on the molecular weight Mw are at least one moietyselected from DOPA (L-3,4-dihydroxyphenylalanine), and aDOPA-derivative.
 7. The method according to claim 1, wherein the atleast one polymer is at least one polypeptide extracted from abyssus-forming mussel.
 8. The method according to claim 1, wherein theat least one polymer is a polypeptide comprising 30-3000 amino acidresidues and tandemly linked peptide repeats comprising 3-15 amino acidresidues each.
 9. The method according to claim 1, wherein the at leastone polymer is a mussel adhesive protein.
 10. The method according toclaim 1, wherein the polymer is at least one protein selected from thegroup consisting of MEFP-1, MEFP-2, MEFP-3, MEFP-4, and MEFP-5.
 11. Themethod according to claim 1, wherein the polymer is oxidized.
 12. Themethod according to claim 1, wherein the coating comprises at least onelayer comprising the at least one polymer, and wherein the coatingfurther comprises at least one layer comprising the at least one ceriumoxide.
 13. The method according to claim 1, wherein the coating is atleast partially applied to an object comprising carbon steel.
 14. Themethod according to claim 1, wherein the coating comprises dihydrogenphosphate.